This invention relates to processes for coating an implantable device or an endoluminal prosthesis, such as, for example, a stent.
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially press against the atherosclerotic plaque of the lesion for remodeling of the vessel wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings which can collapse and occlude the conduit after the balloon is deflated. Vasospasms and recoil of the vessel wall also threaten vessel closure. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, a stent is implanted in the lumen to maintain the vascular patency.
FIG. 1 illustrates a conventional stent 10 formed from a plurality of struts 12. The plurality of struts 12 are radially expandable and interconnected by connecting elements 14 that are disposed between adjacent struts 12, leaving lateral openings or gaps 16 between adjacent struts 12. Struts 12 and connecting elements 14 define a tubular stent body having an outer, tissue-contacting surface and an inner surface.
Stents may be used not only as a mechanical intervention but also as a vehicle for providing biological therapy. As a mechanical intervention, stents may act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small cavities via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in patent literature disclosing stents which have been applied in PTCA procedures include stents illustrated in U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor. Mechanical intervention via stents has reduced the rate of restenosis as compared to balloon angioplasty; restenosis, however, is still a significant clinical problem. When restenosis does occur in the stented segment, its treatment can be challenging, as clinical options are more limited as compared to lesions that were treated solely with a balloon.
Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
Although stents work well mechanically, the chronic issues of restenosis and, to a lesser extent, stent thrombosis remain. These events are affected by, and made worse, by mechanical aspects of the stent such as the degree of injury and disturbance of hemodynamics. To the extent that the mechanical functionality of stents has been optimized, it has been postulated that continued improvements could be made by pharmacological therapies. Many systemic therapies have been tried. A challenge is maintaining the necessary concentration of drug at the lesion site for the necessary period of time. This can be done via brute force methods using oral or intravenous administration but the issues of systemic toxicity and side effects arise. Therefore, a preferred route may be achieved by local delivery of drug from the stent itself. Stents are composed of struts that are typically 50-150 microns wide. Being made of metal, plain stents are not useful for drug delivery. Therefore, a coating, usually of a polymer, is applied to serve as a drug reservoir.
Slotted tube stents are made by laser cutting a solid metal hypotube. Leading stent manufacturers can produce thousands of stents per day. Consequently, the drug coating process, which is added on to the existing stent manufacturing process, needs to be rapid and reproducible. Stents are difficult to coat evenly due to their intricate geometry and small size. Conventional coating techniques fill in the spaces between struts creating webbing and bridging. A versatile method of stent coating is by a spray process that avoids webbing by the application of small droplets.
In order to coat a stent, it typically must be held in some manner. This allows it to be positioned and moved under a spray nozzle in a controlled and repeatable manner. However, holding a stent requires making contact with it. At these contact points, the liquid coating can web, accumulate or wick. After drying, this leads to thick coating deposits at the contacts between the stent and the fixture. These deposits can also attach the stent to the holding fixture, which creates tearing and bare spots when the two are eventually separated. It is desirable that the stent be completely coated on all surfaces with no significant bare spots. It is also desirable that there be no significant defects associated with the fixturing. It is further desirable that a coating process is capable of allowing the coating of a large amount or number of stents at one time.